1. Technical Field
This invention relates to a system for damping and limiting internal forces, accelerations, drifts, etc. caused by external excitations such as earthquakes and wind in structures such as buildings and bridges, and more particularly, to a structural system in which the periods of response, deflection patterns and damping capabilities of the structure are set by the invention in order to dissipate seismic energy and control the internal forces, displacements and accelerations.
2. Description of the Prior Art
In order to protect a structure such as a building or bridge, as well as people who may occupy or be in the vicinity of such structures, seismic force resistive systems have been devised. These systems occasionally include active and/or passive seismic energy dissipation devices. These systems attempt to protect the structure from total collapse and control damage in response to seismic forces created by earthquakes. Typically a local building code sets minimum equivalent seismic forces that must be resisted by the structure based on various parameters. The magnitude and distribution of these forces are generally determined as a function of the structure's mass, its vibrational properties (periods of response), the regional seismicity, the local soil conditions, the type of seismic system, and the importance of the structure.
For conventional structures that include a seismic resistive system, such as a seismic bracing system, predominant periods of response and the damping capabilities are effectively "built-into" the system as determined by its mass and the configuration and material properties of the elements that comprise the seismic bracing system. These elements usually consist of various types of structural walls and/or frames. The period of response of the structure and its damping capabilities affect the internal forces, accelerations, and displacements of the structure in response to a given earthquake.
With conventional bracing systems, the lateral elastic seismic response of a structure is affected by the stiffness of the seismic system relative to the shaking characteristics of the earthquake and the system's damping properties. In general, moderate to large earthquakes, at moderate to large distances from a structure's site, produce larger forces in stiffer structures and smaller forces in softer structures when the soils under the structure are at least moderately stiff. Under the conditions described above, the relative deflections are smaller for stiffer structures and larger for softer structures. These general relationships may not be valid if the structure is located very close to an earthquake fault that ruptures or if the structure is underlain by extremely soft soils.
Generally, a stiffer structure must be made stronger than a softer structure to elastically withstand larger seismic forces. The stiffer structure produces higher internal accelerations that can adversely affect its contents. When a stiff structure is not made strong enough to resist the elastic seismic forces, it sustains structural damage. By sustaining damage, the structure dissipates the seismic energy. Internal forces are limited to the elastic strength of the structure. The structure effectively softens and undergoes larger drifts. These larger drifts are in-elastic deformations that can increase non-structural damage.
On the other hand, a softer structure can generally be made relatively weaker than the stiffer structure and still elastically withstand seismic forces from a given earthquake. As with a stiff structure, when the soft structure is not made strong enough to resist the elastic seismic forces, it also dissipates the seismic energy by sustaining structural damage. The soft structure may undergo large deformations that can adversely affect non structural elements by subjecting them to the structure's deformations.
The ability of a structure to sustain significant structural damage without a significant loss of strength, (i.e., structures with high ductility) is accounted for in the local building code. Structures with high ductility may be designed to be weaker than those with lower ductility.
Supplemental damping may be provided by adding Energy Dissipating Units (EDU) within the bracing elements of a conventional seismic bracing system. The added damping improves the seismic performance of the structure by reducing deflections, accelerations, and structural and non-structural damage.
FIG. 20 graphically illustrates the general relationships between acceleration, period of response and damping for a simplified model that may be used to understand complex structures.
There are several basic relationships of strength, stiffness and ductility of structural systems that affect current design practice. Some of these relationships include:
For a given size and type of structure, an increase in strength usually results in an increase in stiffness; PA1 An increase in stiffness usually results in an increase of seismic forces, internal accelerations (and potential content damage), and a decrease in deformation; PA1 A decrease in strength usually results in an increase in ductility demand or damping demand; PA1 An increase in supplemental damping usually results in a decrease of forces, deformations, internal accelerations, and structural and non-structural damage; and PA1 Relative improvements in strength, ductility and damping of a system usually result in added costs.
A typical arrangement of supplemental damping within a moment frame structure is illustrated in FIG. 1. All or part of the system's lateral static stiffness is a result of the flexural stiffness of the beams 13 and columns 14 that are connected with a rigid or semi-rigid joint also known as a moment joint 15. The braces 10 are added to the frame between each level in order to couple the levels with an Energy Dissipation Unit (EDU) 11. The energy dissipation devices may work by using several mechanisms such as friction, yielding metals, energy absorbing plastics, rubbers, etc, and fluids forced through orifices. These devices (EDUs) may be activated by the relative displacement between each level, by the relative velocity between each level, or by active control methods. The EDUs may also provide additional static stiffness to the frame via the braces. In a second common arrangement, illustrated in FIG. 2, a brace 10 extends diagonally between portions of a frame with an EDU 11 in the middle.
Supplemental damping devices may add substantial costs to conventional seismic bracing systems. To date, the costs associated with the installation of these devices has been a factor in their limited use.
Another prior art system, illustrated in FIG. 3 has an isolation layer under the entire building and is commonly referred to as a base isolation system. The isolation layer utilizes isolators 16, generally in the form of bearings, and controls imparted accelerations and deformations in two ways: by affecting the structure's period of response, since the bearings are relatively soft when subjected to lateral ground accelerations; and by providing damping. Optional supplemental damping devices 11 may be added. The damping devices (those integral with the bearings and supplemental) help to control the structure's deformations, accelerations and forces. The structural system above the isolators tends to be similar to that of a conventional seismic bracing system; however, the isolated structure would tend to sustain less damage during a large earthquake.
Since the isolation layer is soft, the structure experiences large horizontal movements or "drifts" (even with the optional damping devices). These drifts, which are generally between one and two feet, must be accommodated by the various building systems such as elevators, piping, power lines, etc. Additionally, the building must be separated from and allowed to deform relative to the surrounding grade by means of a special covered mote or seismic joint. Base isolation buildings tend to perform better than ones with conventional seismic bracing systems during earthquakes. However, there are significant added costs associated with the installation of a base isolation system that are attributed to the isolators, the mote, and the special building system details needed to accommodate the large deformations. These costs, to date, have limited the use of base isolated systems.
Accordingly, a seismic system that controls loads, internal accelerations, deformations, and structural and nonstructural damage while being economical and non-disruptive to the function of the building is needed.